This invention relates to a method and an apparatus for measuring illuminance distribution and, more particularly, to a method and an apparatus for measuring illuminance distribution suitable to be used with an exposure apparatus for transferring a pattern of a mask onto a wafer at a high resolution.
Recent trend of further miniaturization of a pattern of integrated circuit (IC) or large scaled integrated circuit (LSI) has required a strictly uniform line width of the pattern of the device. Such requirement necessitates uniform distribution of irradiation energy on a transfer plane in the exposure apparatus. The illuminance distribution on the transfer plane has conventionally been measured by moving one photodetector along the transfer plane. This is because of the difficulty in manufacturing a photodetector having a large light-receiving area.
The conventional measuring method using the moving photodetector is disadvantageous in the point that changes in the luminance, with time, of the light source would directly lead to non-uniformity of the distribution of illuminance.
Conventional continuous emission type light sources such as Hg lamps usually cause only a minute amount of luminance fluctuation so that the measurement of illuminance distribution contains only a minute amount of error. However, in order to extend the service life of the light source or to achieve a higher luminance, it is possible to arrange the light source to effect pulse-like emission, e.g. by applying a higher input voltage to the lamp only during the mask pattern transfer, or to use a pulse-oscillation type laser, e.g. excimer laser, as the light source. In the case of using such pulse emission type light source, the conventional method for measuring the illuminance distribution by moving one photodetector along the transfer plane would not assure correct measurement of the illuminance distribution, because the pulses usually contain non-negligible energy fluctuation. Further, while the continuous emission type light source will cause only a minute amount of luminance change, as described above, such minute luminance change will still be non-negligible in respect to precise measurement of the illuminance distribution.